The invention relates generally to a system and process for manufacturing high transition temperature (Tc) oxide superconductor wire. The invention more particularly relates to a system and apparatus for coating, winding and heat treating high Tc oxide superconductor wire.
Many applications of the high Tc oxide superconductors require forming the superconductor into a wire. Current processes include forming a precursor to the oxide superconductor or the oxide superconductor itself into a wire and heat treating the wire to obtain an oxide superconducting wire. Current developments in high Tc oxide superconductor processing have resulted in the manufacture of increasingly longer lengths of oxide superconductor wire with acceptable current carrying capacity. The loading and unloading of oxide superconductor wire is an important step in the processing of the wire, in particular, to move the wire between wire deforming steps, such as pressing and rolling, and oxide superconductor phase-forming steps, such as sintering and annealing. The prior art wire deforming step is typically carried out by feeding the wire from a conventional spool through the deforming step and taking up the deformed wire onto a second conventional spool. Long lengths of wire can be efficiently wound onto the spool. However, the conventional spool is not readily adaptable for use in furnaces and results in inefficient use of furnace space. Further, the wire tends to sag and become distorted (wavy) because of the coiled form of the wire on the spool. Lastly, the multiple overlapping windings on the spool do not permit efficient oxidation and phase transformation of the oxide superconductor.
A further disadvantage to winding the oxide superconductor wire prior to heat treatment is that overlapping contact between portions of the wire results in diffusion bonding of the wire to itself and the mandrel, thereby degrading superconducting properties and preventing the unspooling of the heat treated wire. Silver is commonly used as a protective cladding for the oxide superconductor, in particular because the cladding itself is electrically conductive and does not prevent oxygen diffusion to the oxide superconductor. However, even the silver cladding will diffusion bond to other portions of the silver-clad wire which are in contact during heat treatment.
U.S. Pat. No. 5,140,006 discloses a method and apparatus for coating a silver-clad oxide superconductor wire with a diffusion bond-inhibiting material and taking up the coated wire onto a spool. Rare earth oxides are specifically disclosed as a desirable diffusion bond-inhibiting material and no disclosure of the desirability of removing the material after treatment is disclosed.
It is an object of the present invention to efficiently process high-Tc superconducting oxide wire by increasing the simplicity and efficiency of the method used to load and unload wire during processing, by reducing adhesion of the wire to itself and by maximizing the use of furnace space during heat treatment. It is a further object of the invention to accomplish the above while maintaining adequate solderability of the wire for subsequent processing.
In one aspect, the present invention provides a method of processing an oxide superconductor wire that promotes solderability. The method comprises applying an isolating layer to the surface of the wire, the isolating layer comprising an isolating material and a porosity-inducing component, and heating the wire to produce a removable porous coating. The wire can then be processed (e.g., by heat treating), and at least a majority of the porous coating is removed. Finally the wire is exposed to an environment (e.g., a reducing environment such as hydrogen gas, forming gas, hydrogen chloride gas, hydrochloric acid, carboxylic acid, ammonium chloride, a borax, or a solder flux) which converts any remaining portions of the coating into a solderable metal. The isolating material may be, for example, copper oxide or silver sulfide. The porosity-inducing component may be cellulose, wood fiber, saw dust, graphite, paraffin, polypropylene, polyethylene, or mixtures of these, and may be in the form of an extrudable binder. The layer may be applied by a wide variety of methods, including dip coating, spray coating, coating with an adhesive slurry, electroplating, electrostatic attraction, or coextrusion. The majority of the layer may be removed after processing by passing the wire under a stream of water, chemically or electrolytically etching the wire, abrading the wire surface, passing the wire through an ultrasonic bath, wiping the wire, or applying a vacuum or a blower to the wire. At least a majority of the layer is removed, and preferably at least 75%, 80%, 85%, 90%, or 95% is removed.
In another aspect, the invention comprises a method of improving the solderability of a superconducting wire comprising metal compound particles on its surface, by reducing the particles to a solderable metal. The particles may be, for example, copper oxide or silver sulfide, and may be reduced by exposure to hydrogen gas, forming gas, hydrogen chloride gas, hydrochloric acid, carboxylic acid, ammonium chloride, a borax, or solder flux.
In still another aspect, the invention comprises a composite wire reel adapted to be heat treated in a furnace, comprising a composite wire with a removable porous coating comprising copper oxide or silver sulfide on its surface. The wire is wound in a spiraling fashion to form a flat reel, with each turn of the spiral in alignment with the preceding turn of the spiral to form a pancake-like shape. Such reels are well-adapted to be stacked and heat treated in large batches in a furnace.
In yet another aspect, the invention comprises a method of processing an oxide superconductor wire, by co-winding the wire with a separating layer onto a reel, processing the reel, and then removing the separating layer. The separating layer may be, for example, a high temperature superalloy, copper, nickel, or cigarette paper.
In a further aspect, the invention comprises a method of processing an oxide superonductor wire, comprising applying an isolating layer comprising an isolating material and a porosity-inducing component to the wire. The isolating layer is then transformed into a removable porous coating, and the wire is processed. Finally, the wire is exposed to an environment which substantially converts the porous isolating layer into a solderable metal.